fleetctl: add list-unit-files command

This is functionally equivalent to `fleetctl list-units --fields
unit,hash,dstate' for now; it will start to make more sense as the registry
API and `fleetctl list-units` receive their own changes.

Not really sure on good descriptions so open to suggestions.
My original concern was a bit orthogonal to what you bring up. If we are no longer storing a unit file in the original format provided by the user, the approach to comparing fleet jobs based on unit hashes no longer works. fleet will generate a hash from a serialized list of UnitOption objects, while the user will have the original unit file on disk. Those will not match.

In the scenario you bring up, we will still be able to generate hashes of different unit files that a user could compare, they just wouldn't be able to generate a hash of a local file and compare that to what fleet reports.